It is known that data recorded on magnetic disks are contained inside concentric circular tracks and take the form of a succession of small magnetic cells, known as elementary cells, distributed over the entire length of each track.
In recording on magnetic disks, the present trend is to seek to obtain radial densities of several thousand tracks per centimeter (measured radially) and linear densities that are equal to or greater than ten thousand changes of the direction of magnetization per centimeter (measured along the circumference of the tracks).
A preferred mode of writing data that makes it possible to obtain such recording densities is known as perpendicular recording, where the magnetization in the elementary cells is perpendicular to the magnetic recording layer of the disk. In this mode, the magnetic medium comprising the layer is an anisotropic magnetic medium, having a preferred direction of magnetization, hence known as the direction of easy magnetization, perpendicular to the recording layer.
Generally, the disk (or recording substrate) thus comprises a nonmagnetic substrate on which the magnetic medium for perpendicular recording is deposited, the thickness of this medium being very small, on the order of a micron. On this magnetic medium, a nonmagnetic protective layer is typically deposited, so as to assure mechanical protection for this medium and to prevent corrosion or oxidation.
The magnetic medium for perpendicular recording typically comprises an alloy including at least two simple elements, such as chrome and cobalt, iron and terbium, terbium and gadolinium, etc.
The minimum quality required for perpendicular recording media has to do with the coercive field and the rectangularity of the representative hysteresis curve of this medium in the direction of easy magnetization. The coercive field H.sub.c in this direction must be high enough to avoid any perturbation of the data written on the disk by parasitic magnetic fields. At the same time, the coercive field must not be too high, so that it will be possible to write data by reversing the direction of magnetization in the adjacent elementary cells by means of the magnetic writing field. This field is generated by a magnetic writing transducer associated wth the magnetic disk. Under the conditions defined above, the coercive field has a value that in practice is on the order of a few hundred oersteds. It should also be noted that for high-density data, the reading signal generated by the magnetic transducers for reading these data is proportional to the coercive magnetic field H.sub.c.
Furthermore, the ideal form of the hysteresis curve or cycle of a magnetic medium for perpendicular recording is a rectangle. On this subject, reference may be made to the thesis by Dominique Jeanniot submitted to the Universite Pierre et Marie Curie on Nov. 21, 1983, entitled "preparation et caracterisation de couches minces anisotropes de CoCr et de a-FeTbGd pour enregistrement magnetique perpendiculaire" ("Preparation and characterization of thin anisotropic layers of CoCr and a-FeTbGd for perpendicular magnetic recording"). In effect, the remanent magnetization thus remains elevated, being as near as possible to saturation magnetization.
Among the metal materials preferably used to make perpendicular magnetic recording media are the alloys of chrome and cobalt comprising 15 to 20% chrome and 80 to 85% cobalt. The properties of chrome/cobalt alloy (CoCr) are well known, however, because they have been the subject of many studies cited in the aforementioned thesis, particularly on pages 41, 42, 43 and 128 thereof.
Perpendicular magnetic recording media of CoCr has been obtained particularly by cathodic diode radiofrequency pulverization, performed under partial argon pressure at temperatures on the order of 250 to 300 degrees . An explanation of cathodic radiofrequency pulverization is provided in the aforementioned thesis, in Appendix I, pages 133-151.
It has been possible to confirm by experimentation that beginning at a certain thickness of the CoCr recording medium, on the order of a few thousand angstroms to 1 micron, the anisotropic magnetic properties of the chrome/cobalt alloy deteriorate: The coercive field decreases, and the hysteresis cycle is less rectangular. This deterioration of the magnetic properties of the chrome/cobalt alloy becomes still greater, the more the thickness of the magnetic medium increases. This is due to the fact that the chrome/cobalt magnetic medium is under strain due to internal mechanical stresses inside the medium which tend to deform the layer. The existence of these stresses is due for example to differences in dilation arising between the magnetic recording medium and the substrate after the magnetic medium has been deposited on the substrate. These differences in dilation arise at the moment the recording substrate, as a whole, cools. This modification of the magnetic properties of the perpendicular recording medium brought about by the strains is due to the magnetostrictive effect, which arises at the microscopic level. It will be remembered that the magnetostrictive effect means that for certain materials, the existence of mechanical strains causes a modification of the magnetic properties, and reciprocally, the application of a magnetic field to these materials engenders a mechanical strain. Since, as noted above, the anisotropic magnetic properties deteriorate, the magnetostrictive effect is known as the negative magnetostrictive effect.